A study of the stable tyrosyl radical (Y122) in E. coli ribonucleotide reductase using high frequency (139.5 GHz) continuous wave electron paramagnetic resonance (EPR) spectroscopy is presented. The spectral dispersion achieved at this frequency and corresponding magnetic field strength (approximately 5 Tesla) allows the unambiguous measurement of all three principal g-values (2.00912, 2.00457, 2.00225) from a frozen solution sample. The resolution of proton hyperfine splittings along the canonical positions in the spectrum provides a means to determine the anisotropic hyperfine coupling constants and to correlate the g- and hyperfine principal axis orientations. The effect of hydrogen bonding to the phenol oxygen on the g-values of tyrosyl radical species is discussed, as are the implications of this work with regards to the extension of orientationally-selective EPR techniques, such as ENDOR, to high frequency.